I'm not getting how that bit works. Isn't it just for fixing the page's context within a menu that's already displayed?

I want to know how to display a new menu within the new page.

I've had a search around and can't find anything that actually makes sense to me. The way Drupal's menus work is alien to anything I've done before, so I might need an idiot-proof introduction to how it works. Maybe before/after the BTS meeting on Thurs?